In order to improve the gloss, smoothness and ink holding capabilities of paper, it is often desirable to coat the paper with an appropriate composition. One conventional method of coating paper is to apply a layer of a selected coating composition to one side of a web of paper, dry the layer of composition, and then apply a layer of composition to the other side of the web. A good uniform coating can be obtained on both sides of the web, with different thicknesses and/or other qualities of coating on each side, if desired. The method is usually carried out using separate coaters and is referred to as "off-machine" coating. Drying the coating on one side of a web before applying a coating to the other side of the web is expensive, however, and it is usually impossible to rebuild existing paper machinery so as to produce paper coated on both sides using such a coating method. Generally speaking, insufficient space is provided in existing machinery to allow the installation of the two coating stations and the intermediate drying station required by the method.
Another conventional method of coating two sides of a web of paper is described and illustrated in Swedish Pat. Nos. 347,781 and 348,777. The method practiced by the apparatus of the Swedish patents involves applying a liquid coating composition simultaneously to both sides of a moving web of paper and then passing the web between at least one pair of opposed doctor blades that smooth the coating and control the total thickness of the coated paper. While the method permits control of the total thickness of the coated paper, it is not possible to control the thickness of the coating composition on the individual sides of the web, because any attempt to alter the coating thickness on one side of the paper web will simultaneously produce a corresponding change in the coating thickness on the other side of the web of paper. For example, if the blade pressure is increased against one side of the web of paper, a simultaneous increased in pressure between the web and the doctor blade on the other side of the web will also occur. The reciprocal action of the blades also makes it impossible to coat a web of paper with a relatively heavy or thick coating and still maintain adequate control of the process. In practice, the apparatus, such as described and illustrated in the two Swedish patents, is limited to applying a coating of approximately 12 grams of absolutely dry coating composition per square meter of web on each side of a web.
Streb et al U.S. Pat. No. 3,293,067 is directed to a method of applying a liquid coating to both sides of a web of paper using a roll applicator on one side and using trailing blade coating methods on the other side. As indicated in the patent specification, the trailing blade coating methods include applying the coating composition to the web using a dip roll and then doctoring the composition using an inverted flexible blade spaced from the roll in the direction of travel of the web. In the method of the Streb et al. patent, no drying of the composition applied to one side of the web of paper occurs before the application of composition to the other side of the web.
When coating a paper web using the method of the Streb et al. patent, the coatings applied to the two sides of the web are unequal in thickness and the method offers only limited possibilities for controlling the thickness of the coat on each side of the web, especially with quantitatively large applications. A certain two sidedness occurs in the microstructure of the coat and with large applications of coating composition, there is a continuous risk of obtaining a coated paper with pronounced two sidedness. An increase in the amount of coating composition applied to the side of the web coated by the roll coater will also affect the side of the web which is blade coated, because the increased thickness of the coating on the roll-coated side will produce higher pressures between the doctor blade and other side of the paper web.
If both sides of a web of paper are coated using conventional trailing doctor blade coating methods without intermediate drying, the results are also unsatisfactory. In such a process, after the web of paper is wrapped around a backing roll, the coating solution is applied to the web by an applicator roll, for example, and then smoothed and held to desired thickness by a doctor blade. If the coating applied at a first coating station to one side of the web is not dried before coating is applied at a second coating station to the other side of the web, at least part of the coating applied at the first station will be deposited on the backing roll in the second coating station. As a result, both sides of the web will have an irregular coating due to the splitting of the coating film which will take place when the paper web is separated from the backing roll at the second coating station. The coating that develops on the backing roll at the second coating station, which is difficult to clean effectively from the roll, thereafter causes the coating on the side of the web coated at the second coating station to be uneven due to the higher blade pressures resulting in the zones where coating remains stuck on the backing roll. Linear irregularities in the blade pressure cause an unevenly doctored quantity of coating across the fibrous web, resulting in a product of unacceptable quality.